Resolution of items beyond the limits of optical microscopes due to the diffraction of light has become increasingly important. However, such “super resolution” microscopy techniques have proven lacking in application to certain subjects.
However, techniques have been developed for determining properties for a subject, for example Magnetic Resonance is used for conductive regions. The spatial resolution of traditional Magnetic Resonance Imaging (MRI) techniques is typically dictated by the strength of the applied magnetic field gradients, resulting in hard resolution limits of the order of 20-50 μm in favorable circumstances. Imaging using Magnetic Resonance (MR) techniques has typically relied on the ability to encode spatial information in the frequency or phase of the precessing nuclear spins. In MRI, this process is achieved through the application of magnetic field gradients, which has led to a plethora of applications in the health field and in the materials sciences alike. The resolution limit in conventional MRI is often hardware-related. For clinical MRI, for example, this limit is typically dictated by the maximum gradient strength. Intrinsic sample properties, such as chemical shift dispersion or fast relaxation, particularly in rigid samples, are further frequently-encountered constraining factors.
The spatial variation of the radiofrequency (rf) field has also been used to perform imaging using nuclear spins. In the most straightforward case, spatially resolved information can be obtained from a given volume of a sample by placing it within a coil region with large rf field variations. Surface coils are particularly useful in this regard as they have a well-defined rf field profile that can penetrate the surface region of a sample to yield localized spectroscopic information, with clear uses for in vivo applications. Classes of ‘depth pulses’ and pulse sequences were later developed to be used in conjunction with surface coils to further enhance the spatial selectivity. These experiments form part of a larger class of MR imaging methods that can be used to study planar samples.
In magnetic resonance imaging, localization is performed with the help of magnetic field gradients. In conductors, there is an intrinsic spatial dependence of the rf field due to induced eddy currents on the surface of the object that oppose the propagation of the wave into the medium. Electromagnetic radiation decays exponentially when it enters a conducting region with a characteristic length, called the skin depth,
                    δ        =                              1                          πμ              ⁢                                                          ⁢              v              ⁢                                                          ⁢              σ                                                          (                  Equation          ⁢                                          ⁢          1                )            where ν is the frequency of the field, μ the permeability of the conductor and σ its conductivity. This effect has profound implications for the sensitivity of magnetic resonance (MR) techniques, which rely on radio frequency (rf) fields to excite and detect precessing spins from within conducting regions.
A key feature Equation 1 above is the dependence on ν−1/2 which means that at higher frequencies (corresponding to experiments performed at higher magnetic fields) δ is reduced. For example, δ=12.3 μm for nuclear spins of Lithium-7 (7Li) in metallic lithium at a magnetic field of a 9.4 T (larmor frequency, νn=155 MHz) while Lithium-6 (6Li) nuclei in the same sample will have a larger effective skin depth, δ=20.0 μm because of the lower gyromagnetic ratio of this isotope and therefore lower larmor frequency (νn=59 MHz). For a corresponding electron spin transition, GHz frequencies would be relevant, and the skin depth would be in the range δ≈1 μm.
Studying commercial battery designs under their typical operating conditions using conventional analytical tools has proven to be very difficult due to the large size, complicated structure and material properties of these devices. Due to these limitations, most studies have been restricted to specialized cell designs with properties amenable to study using specific techniques. These restrictions have meant that the investigation of performance and failure mechanisms in batteries is still performed destructively by cycling multiple cells and taking them apart at critical points to analyze changes that have occurred. This process involves considerable time, effort and expense. Moreover, physical and chemical changes occurring when the cell is taken apart can compromise any information obtained.
Batteries are a crucial enabling technology in many important energy solutions integral to advances in portable electronics, electric vehicles and grid storage. Continued demand for batteries with high energy capacity and the desire to quickly charge and discharge the devices present a number of formidable engineering and scientific challenges. Ensuring device safety is an important consideration, which needs to be addressed with care. Several industry leaders have experienced unforeseen setbacks due to battery and cell malfunctions, such as most recently, for example, seen in the Samsung Note 7 devices or in the iPhone 8 swelling issues. One major reason for the recurrence of such problems, and for the slow progress in battery technology is the difficulty in tracking defects inside the cells during operation in a nondestructive fashion.
X-Ray CT is a successful technique for scanning electrochemical cells, but it is relatively slow, and thus usually not applicable for high throughput or in situ applications. Furthermore, X-Ray CT provides diagnostics mostly of the denser components of a cell, and does not offer insights into subtle chemical or physical changes of the materials inside. A recently-developed acoustic technique appears to be a highly promising methodology for the non-destructive characterization of cell behavior throughout the cell life, and is currently being investigated for its sensitivity to important cell behavior.
Magnetic Resonance (MR) techniques have been developed to measure several different cell properties. A fundamental limitation that is difficult to overcome under typical operating conditions is that conductors are not transparent to rf irradiation. Often the cell casing is made of conductive material, such as polymer-lined aluminum in pouch or laminate cells, but also the electrodes preclude the use of conventional MR for realistic or commercial-type cell geometries. Nonetheless, MR has provided important insights into electrolyte behavior, Li-dendrite growth, and other electrochemical effects by the use of custom-built cells, which allow convenient rf access.
The prospect of applying magnetic resonance techniques (e.g., NMR and MRI) to commercial batteries is restricted because almost every cell design is encased in a conductive material, for example, solid stainless steel, aluminum, aluminum-laminated films used in pouch cells, etc. The radiofrequency (rf) fields used in typical magnetic resonance experiments are incapable of penetrating the conductive material (i.e., metallic layer) in order to excite and detect the nuclear magnetization.
A need exists for improved technology including a microscopy technique that exploits the intrinsic changes imparted on the rf field when it enters a good conductor, rather than using intrinsically designed magnetic field profiles or stray magnetic field gradients. Conducting systems offer unique challenges compared to those tackled by the ‘depth pulse’ and related techniques described above, due to the fast T1 and T2 relaxation of the nuclear spins and the intrinsic shape of the rf field profile. Embodiments of the present application, termed Slice Microscopy in Conductors (SMC), exploit these traits and provide the ability to select slices within the objects. A need also exists for improved technology capable of applying magnetic resonance techniques to measure physical and chemical changes in conducting structures, including batteries encased in a conductive material.